1. Field of the Invention
The invention relates to a pipe joint having a sleeve element and a spigot element.
2. Discussion of the Prior Art
Pipe joints serve for connecting tubular elements, for example to form pipelines, which deliver, in particular, pressurized fluids, for example gas or oil. In the search for oil or gas, these pipe joints are also used in lengths of casing pipes or delivery pipes.
The pipe connections in question here are usually made using threads, an external thread on a spigot element and an internal thread in a sleeve element, which are screwed together. As far as the definitive positioning after the screwing-together operation is concerned, a distinction is drawn between pipe joints with a marking made on them and pipe joints which have an additional abutment element such as inner shoulder or outer shoulder. This shoulder element may additionally assume the function of a seal. The shoulder essentially comprises annular surfaces which are arranged, for example, at the free end of the spigot element and on the inside of the sleeve element and are pressed firmly one upon the other by the screwing-together operation. In the case of an inner shoulder, the spigot element is compressed between the shoulder and the thread and the sleeve element is expanded between the shoulder and the thread; In the case of an outer shoulder, the spigot element is expanded between the shoulder and the thread and the sleeve element is compressed between the shoulder and the thread. The sleeve and spigot stressing caused by the contact pressure in the shoulder is predominantly absorbed only by the small number of teeth at the end of the thread which are located adjacent to the shoulder surfaces. The remaining thread teeth are only subjected to an extremely small degree of loading. Accordingly, the loaded thread teeth are highly stressed. This high stressing in the thread is increased by external loading on the connections, in particular by axial tension and/or compression, but also by internal and external pressure and by bending, with the result that the overall stressing can reach a value which is greater than the permissible yield strength. Axial tension can eliminate the prestressing in the abutment, and thus its action can eliminate the positioning and sealing, and axial compression can result in plastic deformation in the shoulder region and, in the case of subsequent tensile loading, can render the positioning and sealing functions ineffective.
In order that the forces acting on the sealing surfaces (abutment) can be distributed as reaction forces over the largest possible number of thread teeth German reference, DE 34 31 808 A1 discloses a specially designed pipe joint. In this proposal, the pipe joint has an intermediate threaded section between the ends of the spigot thread and sleeve thread, in which the two elements have the same pitch. In the end sections, the pitches of the spigot thread and sleeve thread differ, to be precise in dependence on the position of the abutment. If the annular sealing surfaces (abutment) are located on the inside of the pipe, then the thread of the spigot element has a greater pitches than the thread of the sleeve element; if the sealing surface is located on the outside of the pipe, the opposite is true. The proposed arrangement is intended to make it possible for the reaction force originating from the abutment to be transmitted uniformly over the thread teeth. However, loading, such as axial compression and bending, is further introduced largely into the abutment, which is already prestressed to a considerable extent by the elements being screwed together. It is pointed out at the end of the description of the above-mentioned reference that the design explained above can also be used for a pipe joint with a multiple-start thread.
WO 96/07044 discloses a pipe joint in which the loading, such as axial compression and bending, is absorbed predominantly by the thread rather than solely by the abutment. For this purpose, one element, i.e. the spigot or sleeve, has a constant pitch throughout and the element screwed to it is provided with two sections which extend in each case from the ends of the threaded length and likewise have a constant pitch within the section, although this pitch is different from the first-mentioned element. The sections have an offset with respect to one another in the center, with the result that an in-contact narrow tooth or an excessively wide gap is produced in the transition region between the sections, and, outside the offset, the toothed gap in one element is wider than the tooth of the other element to the extent that the guide flank on one side of the offset is braced against the load flank on the other side of the offset, maximum bracing taking place in the center of the thread and decreasing toward the edges of the thread. This design is based on the fact that a difference in the causes the load flank in one threaded region to be braced against the guide flank in the same, or in another, region.
Extensive tests with this pipe joint have shown that the desired effect for bracing the load flank against the guide flank may, in unfavorable cases, be insufficient if there is unfavorable pairing of the differences produced during the production of the threads.
The object to the invention is to provide a pipe joint which can easily be produced and by means of which all external loading on the connection, in particular operating torques, axial compression and bending, is absorbed predominantly via the thread, even taking account of the differences, which are unavoidable during the thread production, from the ideal configuration, the axial tensile loading and internal compressive loading not being adversely affected.
Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in a pipe joint comprising a sleeve element and a spigot element. One of the elements having a thread with a constant pitch and the other of the elements having more than two sections with a pitch which is constant within the section but different from the pitch of the one element. The pitch difference at least of one section is in an opposite direction, in terms of sign, to a remainder of the sections of the other element, and the pitch difference of one section is at least double that of the adjacent sections. In a screwed together position of the sleeve element and the spigot element, in the section with the large pitch difference the thread plate and the thread tolerances are compensated for and the load flank of one corner tooth of the large pitch different section produces the basic bracing together with the guide flank of the other corner tooth. The two adjacent sections build up the thread bracing with at least one of load and guide flanks until a desired thread torque is reached.
In the case of the pipe joint designed according to the invention, irrespective of whether the thread is a single-start thread or multiple-start thread, one element, preferably the spigot element has, in a known manner, a constant pitch extending over the entire threaded length; in the case of a multiple-start thread, said element has an identical, constant pitch. The great advantage of this is that a damaged spigot element can be recut by relatively simple means. If the sleeve element, which is of a special configuration and is technically very complex to produce, is damaged, it is exchanged. However, the idea of the invention likewise permits a special configuration of the spigot element. The idea of the invention can also be. readily applied to integral joints.
The design of the sleeve element is such that axial bracing is achieved within the thread in the screwed-together position. For this purpose, the sleeve element preferably has more than two sections with a pitch which is constant within the section but is different from the spigot element, it being the case that the pitch at least of one section is in the opposite direction to the rest of the sections and the pitch difference of one section is at least double that of the adjacent sections. This achieves the situation where in the screwed-together position, in the section with the large pitch difference, the thread play and the thread tolerances are compensated and the load flank of one corner tooth of this section, with the guide flank of the other corner tooth, produces the basic bracing. The two adjacent sections, with their load and/or guide flanks, build up the thread bracing until the desired thread torque has been reached.
It has proven particularly favorable if the sleeve element has four sections, of which one section has a smaller pitch than the spigot element screwed thereto, and the three successive sections have a greater pitch than the spigot element screwed thereto, the lead of the spigot element serving as the reference pitch. This previously described arrangement applies, in general, to the arrangement of both an interior and/or exterior abutment. By way of the section with the smaller pitch, the load-flank backlash is eliminated, or vastly reduced, in the direction of the thread end.
According to the invention, the extent of the individual sections is also important. Optimum values are achieved when the section with at least a double pitch difference extends at least over three, preferably over four, helixes and the two adjacent sections extend over two or more helixes. The section located in the end region extends at least over three helixes.
The thread may be designed to be conical or inclined in any desired manner. The tooth shape itself is of minor importance. The design of the threaded connection according to the invention achieves the desired effect in terms of the bracing and the associated distribution of the loading over a plurality of thread helixes, also taking into account the tolerances during the thread production.